1. Field of the Invention
The present invention relates to a separator for a battery and a method for producing the same, more particularly to a separator for a battery having a function for avoiding dangers caused by overheat of a high-performance battery such as a lithium battery, etc., and having excellent film shape maintaining properties even at a high temperature.
2. Description of the Related Arts
In recent years, a compact battery of high performance has been required as a portable compact device such as a lap top computer, a mobile telephone, a VTR camera, and a liquid crystal television, is developed. In accordance with this, a lithium battery comes to public notice and is practically used since it has a high electromotive force, a high energy, a small self discharge, and the like.
The battery generally has a separator for preventing a short circuit between positive and negative electrodes. A polyolefin-based organic porous film is used as a separator in the lithium battery in practical use at present (Japanese Laid-Open Patent Publication Nos. 5-25305 and 8-138643).
The polyolefin-based organic porous film is used to restrain a battery reaction when the battery performs unusually. Namely, when a large electric current flows through the lithium battery caused by an external short circuit, a misconnection of the positive and negative electrodes, etc., the lithium battery is heated and an internal temperature of this battery is excessively increased. At this time, this organic porous film is melted and its micropores are clogged, thereby preventing migration of ions in an electrolyte. Thus, the battery reaction is stopped and further heating is restrained.
Such a function is called a shutdown function and is indispensable to the separator of a lithium battery. The lithium battery is designed such that the shutdown function is fulfilled at a temperature close to 130.degree. C. when using polyethylene and a temperature close to 160.degree. C. when using polypropylene as the separator.
However, this shutdown state can be continuously maintained only when micropores of the separator are clogged by a melted substance of polyolefin as a material of the separator. When a large heating state is continued, the separator is completely melted, so that the electrodes come in contact with each other and a short circuit current flows through the inside of the battery. Thus, there is a danger that the heating state is further continued and the battery is further overheated.
As mentioned above, the state in which the separator is completely melted is called meltdown.
As disclosed in Japanese Laid-Open Patent Publication No. 8-111214, a separator for a battery comprising a laminated structure of a glass porous body and a polyolefin porous body is proposed so as to prevent this meltdown.
However, in this case, when the temperature of a resin layer (polyolef in) attains to a complete melting temperature, an internal pressure of the battery is increased and force for expanding the glass porous body is applied thereto. Since the glass porous body used here is a nonwoven fabric of glass fibers, the glass fibers are simply entwined with each other. Therefore, the clearances between the fibers are increased and cracks generate, resulting in a flowing of a short circuit electric current through the battery and also overheat of the battery.